I was wondering which types of wheels teams have found to be the best at shooting fuel this season. Also, how do you guys maintain your angular momentum with your shooter? Do you just use more than one motor, or do you have heavy flywheels or other heavy materials (steel) in your shooter system?
(The correct term is “angular momentum” (denoted L usually). “moment of inertia” is a mass property, denoted J. L = J*omega (omega is rotational velocity))
You may be asking yourself the wrong question.
A flywheel adds angular momentum to the system, but each ball will still slow it down. Your team will need a motor and flywheel combination such that the motors can add energy back into the system at the rate you take it out. Then you can maintain your shot distance.
The calculations of rotational inertia, and the amount of energy each ball removes are pretty straightforward. Your high school physics teacher can help you with them. Then you need to know the rate of fire you are attempting so you can know how much energy will be removed from the system how quickly (power). Then you can figure out how many motors you will need.
So:
- Figure out where you want to shoot from on the field.
- Figure out the energy you need to impart to each ball to get it to fly that distance.
- Figure out the rotational speed your system requires.
- Figure out the gear ratio of a motor that can easily handle running that speed at only 60-70% duty.
- Figure out your rate of fire.
- Figure out how many motors it will require to maintain that rate of fire (at 60-70% of motor output).
- Add as much weight as you can afford to a flywheel to give the motors time to ramp up their output after the ball removes some energy.
In each step, make sure you give yourself at least a 20% margin of error. After all, you’ll be using simple Newtonian physics and probably won’t account for vibration, misalignment, and all the other energy-sapping things that work their way into the system.
It can grow more complicated as well - if you have an accelerator of some sort you only need to add a partial amount of energy at the shooter, etc. This system is an ideal one to set up in an excel spreadsheet. Someone knowledgeable and a little crazy about physics should be able to throw the whole thing together in a day or two.
Then you’ll have to worry about compression and how much energy you’ll have to rob from the system to make sure the balls don’t slip. This is the experimental portion where you simply have to build a prototype and build up the back wall a 1/16" at a time to find out.
Spreadsheets and simple computer programs (see what Ether throws together quickly) are some of the most powerful tools in an engineer’s tool belt. Along with a good spellchecker.
We use two light weight but high MOI Steel flywheels. The idea is to cut weight by placing all the weight at the very edge of the flywheel but keep the same MOI of a much heavier dense flywheel. This allows us to shoot 30-40 balls with little to no drop in the momentum of the wheel shooting the wheels itself. The key balance you need is the number of motors you have and being able to consistently stay within maybe 100 RPM change per shot fired and at 4-10 balls per second that small change in overall RPM can add up fast
We’ve got two of these bad boys on our shooter axle: http://imgur.com/oIdHZOL
It’s 3" or 3.25" OD 12L14 steel
I know that the mass of the spinning system is really important to keep rpm’s, but my team now knows that the power going into it is way more important.
Team 1986, with it’s amazing 58-ball autonomous has four 775-pro motors turning their shooter wheel. I think they’re 347 watts each, 18,000 rpm. so about 1400 watts. I understand they originally had five, but decided for whatever reason that the fifth wasn’t needed.
They drive over to the bin and empty it into their 'bot, then shoot 58 balls in the remaining time. So if I estimate that they can drive and empty the hopper in say four seconds, then they have 11 seconds left, shooting about 5.3 balls a second. With amazing accuracy.
My team, 1108 proved we could shoot accurately with a mini-cim, (215 watts, 5840 rpm) and we can accurately shoot about one ball every 1 second. We can empty the ten balls pre-loaded balls in the auto period and then drive forward. But that’s all we expected to try for. Didn’t foresee the huge ball counts in auto.
We have two six inch KOP wheels as our shooter and they’re going something like 3000 rpm on a steel axle. It actually has good momentum. But we now know momentum isn’t that important. But it’s also important to note that four 775 pros, running so much higher rpm, will probably have a significantly higher angular momentum than a single mini-cim too.
We’re done with our season now (no champs) but for the next time this robot competes in the off-season, we hope to have more power on the shooter.
Would also like to consider using slip-ring contacts to install motors in the middle of the shooting wheel to power it. This way the motor mass is also the momentum mass. Maybe that’s a crazy idea.
Also, if you’re going to put huge power to your shooter, you’ll need an encoder and a PID to keep the rpm constant. Running a motor without feedback will not keep your speed nearly as well.